Apparatus for purifying fibrous suspensions



Aug. 8, 1950 F. BLlKsTAD ET Al. v

APPARATUS FOR PURIFYING FIBRoUs sUsPENsIoNs Filed Jan. 10, 1946 Patented Aug. 8, i950 y. APPARATUsFoR PUmF-YING FraoUs.

" sUsPENsIoNs Finn Blikstad and Erling Pedersen,`

" Sarpsborg! Norway Application January 1o, 1946, serial No. 640,186 v I n Norway January 110, 1945 2 Claims. ('Cl. B2-28) In all cellulose or paper factories a screening apparatus is present, the task of which is to screen the pulp, i. e. removelall impurities in the fibrous suspension. One may distinguish between two kinds of impurities, namely:

(1) Impurities having a greater specific weight than the bres. l

(2) Impurities having about the same specific weight as the fibres.

In order to remove as muchas possible of the rst mentioned impuritiesmost factories use a sand-trap, i. e. a trough or Ynarrovvbox in which the pulp shall ow with low velocity and low concentration in order that heavy impurities such as sand, coal, rust, iron, metal, etc. shall have time to deposit on the bottom of .the sand-trap. It is nevertheless diiiicult for the impurities to sink because they are kept floating byvvbres andy fibre lumps, which is especially prevalent if the pulp concentration is high, for Ainstance above 0.4%.

From the sand-trap the pulp is passed to the screen proper, the task of which vis to remove also the impurities mentioned under (2).

To a great extent ordinary fiat screens, which have a low power consumption per ton pulp and give a fairly pure product, arev used. The flat screens, however, have a :low capacityand require a large space.

The present invention has for its. object novel features' in a method and apparatus for screening pulp in which, as compared ,with the. ordinary fiat screens, as well a greater purity of the screened pulp as an increased capacity per square meter of the screen surface are obtained.

. Accordingtov the invention, a combination `of a sand-trap and oneor more flat screens located on top of the sand-trap is used. Theeffect of the sand-trap is, according to another feature of the invention, increased by subjecting the flowing brous suspension in the, sand-trap trough. to vibration by means ofthescreenfmember. itself,

which as mentioned is located on top ofthe sand-l trap. A sand particle y sinks. rapidly in clear water, while in a fibre suspension, `as mentioned above, it becomes suspendedf in fibres yand iibre lumps. impurities are shaken out of vthe bres andsink substantially morefrapidlyto the bottom. In a sand-trap, the portion of the lpulp which is nearest to the surface is most free of heavy impurities.. Therefore the screen member according to the invention, arranged in such a manner that the uppermost layer of thefflowing .fibrous sus; pension is supplied to'the 'Screenplateg which By means vof vibrations the heavy-v is obtained in the simplest way by causingl thej pulp to pass the screen plate in an `upward direction, thus contrary to the operation of l'ordi nary flat screens. y

A sand-trap operates best with low"owing velocity, because the impurities thenhave timev to sink to the bottom. On the other'han'd a screen operates ata disadvantage if the move-l ment of the pulp along the screen plate is slow, because then an accumulation of pulp, lumps and impurities immediately beneath the screen plate takes place, so that the capacity of the screen and;

the purity of the screened pulp are reduced. The pulp must therefore flow rapid-ly along the screen plate so that turbulence and a good agitation of the unscreened pulp are obtained. In order to obtain this a partition wall may be mounted in the sand-trap trough a short distance below the screen plate in such a manner that thepulp flows slowly in the lower portion of the sand-trap, turns at the end of same, and flows rapidly` in the opposite direction in the upper portionbetween the screen and the `partition wall. InI order to increase the effect ofr the sandftrap, the partition wall itself or its attachmentk ,should be constructed in such a manner thatit ltransmits the vibrations of the screen also to the, pulp below the partition wall, andthebottom of the sand-trap also should be so constructed I or mounted that vibrations in vertical direction are hindered the least possible.

The screen member proper .used in the arrangement according to the invention, is a flat` screen shaped as an open trough` in whichthe bottom is constituted by the. screen yplate itself.`

circular or elliptic, i. el the' movement ofthe screen is elliptic in a vertical plane perpendicular to the shaft.A The horizontal component of movement causes, however, such splashing` on the outside of the screen thatit is diiiicultv tox avoidsplashing 'of unscreened pull? over dinto the screened pulp, and'besides easily causes'pre'# cipitation of resin and consequently spots in the pulp.

It is therefore an important feature of the invention that the vibrations take place substantially vertically. The vibrations may of course be produced by means of electromagnetic or hydraulic means, but it is especially practical to use two or more rotating shafts having equally large eccentric weights which are geared together so that the shafts rotate synchronously in opposite directions, and so that the horizontal components of the forces are eliminated, whereby only vertical vibrations take place.

The suspension or attachment `of the screen should be resilient in vertical and preferably rigid in horizontal direction, which may be obtained by attaching the screen to a horizontal A.plate spring on each side of the screen, or in any other suitable way.

The screened pulp which after having passed the screen plate accumulates in the interior of the trough .may either leave the screen through an opening in one of the sides of the trough, which is connected with a corresponding opening in the sand-trap by means `of a iiexible sealing, or also by means of a Siphon, which is especially advantageous. A

In case ordinary flat screens are not provided with special regulators, it often occurs that a part of the screen plates becomes bare if the pulp supply decreases. These bare spots also sometimes occur even when regulators are used. Thereby air is admitted beneath t'he screen plate, so that splashing and foaming occur, which has the eiect that resin more easily is deposited in the slits. Through the use of a Siphon this is not possible, because the sip'hon is not capable of removing 'allpulp from the screen plate but lets lo'o'se when the stand has become so low that air is not admitted to the siphon. 'Ihe liquid level of the pulp is thus automatically held at a constant height by means of the siphon.

The unscreened pulp outside the screen stands higher than the screened inside, i. e. it has a certain over pressure which causes the pulp to flow throu'ghthe slits. y

order that the screen shall operate under optimal conditions, the over pressure should not exceed a predetermined limit. Because the level of the screened pulp by means of the siphon kept within predetermined limits, it is in this screen only necessary to see to it that the level of the unscr'eened pulp also is kept within predetermined limits, which may be obtained b y means of a level regulator which controls the pulp supply to the screen.

The height of the optimal over pressure is about 3-10 cm., on the average 6 cm., which should be constant at all places of the screen plate. The "flowing pulp below and 4above the screen plate is subjected to flowing resistancewith consequent fall of pressure, which may amount to a few cm. In order that the over pressure shall be approximately equal at all places, the screened as well as the unscreened pulp should therefore flow in the same direction. -Besides the cross-sections, respectively the inclination angle of the screen plate, should be selected in such away that the pressure loss on both sides is about the same.

Even if an individual screen is an independent unit, it is practical to arrange more screens in succession in a longer sandntrap trough having a smaller slit width in the last screens in the series, because the concentration of impurities beneath the latter becomes great. In order to liberate the impurities beneath the last screen as much as possible from fibres, water may be added at the end of the last screen. This would cause the impurities which otherwise are conveyed by the pulp flow, to accumulate beneath the screen if the impurities are not removed vby means of a mechanical device. Such a mechanical device is described in the following, with reference to the drawing.

An embodiment of the invention is described in the following with reference to the drawing, in which:

Fig. 1 shows a longitudinal section of an apparatus for carrying out the method according to the invention, and

Fig. 2 shows a plan view of the same.

Fig. 3 shows a cross-section of the apparatus.

Fig. 4 shows a longitudinal section of the last screen and also illustrates the operation of the scraper device. l

In the drawing two screens are shown, which are mountedin succession in the direction of flow of the pulp. A

The pulp is supplied through a pipe I, provided with a regulating register which is operated by means of a level regulator 2, which keeps the level of the unscreened pulp between two screens constant. From the p'ipe'l, the pulp arrives at the lower part of the sand-trap 3, in which the pulp iiows slowly and is subjected to vibrations from the screens themselves. The bottom of the sandtrap is tight and preferably resilient. At the end of the sand-trap the pulp flow turns and ows thereafter on the upper side of the resilient partition wall 4, below the screens proper. Here the cross-section is substantially smaller in order to obtain more rapid flowing and consequent turbulence The screened pulp passes the vibrating screen plate 5 in an upward direction, and flows on the upper side of the screen plate in the same direction as beneath the same towards the siph'on B, which carries the 'screened pulp over the longitudinal 'side of the trough1 and the sand-trap down into a lower channel 'l vfor screened pulp. Each si'phon is connected to 4a vacuum conduit 8, having la suiciently y'great under pressure to secure the operation of the Siphon.

The details of the screen appear -from Figs. 3 and l. The screen trough proper is suspended in va plate spring 9 on each longitudinal side. Across the screen a bearn Ill is mounted, which carries the motor 'Il and the gear-case l2, with two rotating eccentric weights i3, which rotate in opposite K "cti'ons and synchronously, because they are provided with inter-engaging teeth. f

Y It is 'therefore sufficient that the 'motor only drives` the one shaft. fter the lastscreeh a speial scraper 'device is mounts@ the task of whichis to convey the impurities to the discharge pipe I4 (see Fig. ll, in spite of the fact that water isadded through the sprayer pipe i at the 'eri'd of the screen, with oonsequent counter#flow The scraper consists of one or :more transverse lists which are turnable about a longitudinalaxis, and whichlv all move backv and forth, When the scrapers movetowards the outlet, their ylower edges contact the partitionlwall 4, while on their returningn movement `v(see Fig. x4.) they are posi: tioned substantially horizontal. It is practical that all Scrapers are interconnectedv and move equally. rlhe turning movement is by way of example effected by providing one of the Scrapers with a roller [E ,(rs'eev Fig. 4 A,at the right-hand bottom), secured at both ends of the Iscraper lina `ward movement.

predetermined distance from the turning axis o f the latter, and which during the reciprocating movement rolls on a guiding rail H, which is provided with a hinge on the downwards bent part adjacent to the outlet, so that the guiding roller under the foreward movement, when positioned below the rail, lifts the movable part of thevrail and then arrives on the upper side on the back- The reciprocating movement may be obtained by means known per se, such as eccentric, crank shaft, hydraulic piston, etc.

Among the advantages which are obtained by the method and apparatus according to the invention, as compared with ordinary flat screens, the following are to be mentioned:

No additional need of space because the screens are mounted on top of a sand-trap which almost always is present. A capacity of up to three times the capacity of ordinary flat screens is obtained, owing to the use of the vibrations described. The vibrations are transmitted partly to the sand-trap, so that the purifying power of the latter is substantially improved.

The screened pulp passes the slits in an upward direction, i. e. it is the uppermost layer of the pulp ow containing fewer heavy impurities which passes the screen.

The short pressure pulsations have the effect that the impurities are not so easily pressed into the slits.

During the cleaning of the screen plate the impurities are flushed back in the unscreened pulp, contrary to the operation of ordinary flat screens.

While ordinary flat screens have a thickening effect, i. e. the screened pulp is thicker than the unscreened, the contrary is the case in the screen according to the invention. It is therefore in itself unnecessary to thin the unscreened pulp after one screen before it is supplied to the next screen.

Because the screened pulp is suitably removed by means of a Siphon, the screen plate can never become bare so that foaming and splashing, and besides a, precipitation of resin in the slits, are avoided.

Because the vibrations take place vertically, splashing on the end surfaces of the screen is prevented.

The most favorable conditions of operation are secured through automatic regulation of the pressure difference between screened and unscreened pulp. Owing to a special mechanical device for removing the impurities, there will be practically no loss of fibres.

We claim:

1 Apparatus for purifying fibrous suspensions, comprising in combination a sand-trap, a vibra- 6 table rlat screen located on top of the sand-trap, means for vibrating the screen substantially only in a vertical direction and at a high frequency, said vibration means comprising a pair of eccentric weights which rotate synchronously and in opposite directions, a vibrations-transmitting partition wall which divides the space between the screen and the bottom of the sand-trap into an upper and a lower portion and which at one end of the sand-trap provides a passage between the two portions, the upper portion having a substantially smaller cross-section than the lower portion, and means for moving a current of brous suspension serially through the lower porr tion, the passage, and the upper portion and into contact with the screen.

2. Apparatus for purifying brous suspensions, comprising in combination a sand-trap, a vibratable fiat screen located on top of the sand-trap, means for vibrating the screen substantially only in a vertical direction a vibrations-transmitting partition wall which divides the space between the screen and the bottom of the sand-trap into an upper and a lower portion and which at one end of the sand-trap provides a passage between the two portions, the upper portion having a substantially smaller cross-section than the lower portion, and means for moving a current of fibrous suspension serially through the lower portion, the passage and the upper portion and into Contact with the screen.

FINN BLIKSTAD. ERLING PEDERSEN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 59,510 Krom Nov. 6, 1866 703,682 Vrooman et al July 1, 1902I 2,144,382 Lincoln et al Jan. 17, 1939 2,228,852 Shafer Jan 14, 1941 2,338,523 Lincoln et al. Jan. 4, 1944 2,406,618 Lindgren Aug. 27, 1946 FOREIGN PATENTS Number Country Date 496 Great Britain of 1910 1,079 Great Britain of 1884 14,557 Sweden Aug. 16, 1902 15,070 Austria Feb. 10, 1904 44,641 Sweden Oct. 16, 1918 222,077 Germany May 18, 1910 659,799 Germany May 14, 1938 703,250 Germany Mar. 5, 1941 

